Human and simian immunodeficiency viruses exhibit remarkable biological differences in their rate of replication, their host range, and the extent to which they induce cytopathic effects during infection. However, the mechanisms that underlie these biological differences are unclear. This proposal describes the characterization of variants of SIVmac derived from a single molecular clone that exhibit markedly different biological properties. Importantly, these changes are also associated with striking biochemical, functional and morphologic differences in their envelopes. This project plans to develop this SIV model further in three specific areas. First, the molecular determinants that are responsible for these biological and biochemical differences will be identified using PCR, infectious molecular clones, and chimeric viruses. Second, studies will focus on mechanistic questions concerning the interaction of the virus with the host cell. The changes in envelope that produce high CD4 binding affinity will be mapped genetically, and quantitative assays will be used to measure conformational changes in these molecules to systematically evaluate the structural and functional alterations involved. In addition, the analysis of the TM cytoplasmic tail of the SIV variants has identified a region that is hypothesized to modulate surface expression of envelope on infected cells by regulating the rate of constitutive endocytosis for these protein. This putative signaling element will be characterized and its contribution to the biological changes of the SIV variants will be determined. Third, information derived from this model will be applied to other experimental systems of SIV and HIV where closely related molecular clones with divergent biological properties are available. Specific questions will also focus on the role of the cytoplasmic tails in affecting endocytosis and trafficking of glycoproteins and the contribution of these structures to the biological properties of the viruses. In conclusion, by Ideveloping an experimental model in which molecular information is correlated with distinct biochemical differences in envelope, it is the expectation that this project will provide new insight into both the structure and function of SIV and HIV envelope glycoproteins and contribute to an understanding of their role in pathogenesis both in vitro and in vivo.